August 15, 2013

Researchers Observe Captive Apes Diving And Swimming

The first video-based observation of swimming and diving apes has been provided by two researchers from Wits University and the University of Bern. Instead of the expected dog-paddle stroke used by most land mammals, the apes used a kind of breaststroke. The study, published in the American Journal of Physical Anthropology, suggests the swimming strokes peculiar to humans and apes might be the result of an earlier adaptation to an arboreal life.

Zoos have used water moats for years to confine chimpanzees, gorillas or orangutans. Apes often drown when they venture into deep water. Scientists have argued this revealed a definitive difference between humans and apes: humans enjoy the water and are able to learn to swim, while apes prefer to stay on dry land.

This distinction, it turns out, is not absolute. Renato Bender, PhD student in human evolution at Wits University’s School of Anatomical Sciences worked with Nicole Bender, evolutionary physician and epidemiologist at the Institute of Social and Preventive Medicine at the University of Bern, studying a chimpanzee and an orangutan in the US that were raised and cared for by humans. The apes have learned to swim and dive.

“We were extremely surprised when the chimp Cooper dived repeatedly into a swimming pool in Missouri and seemed to feel very comfortable,” said Renato Bender.

The researchers stretched two ropes over the deepest part of the pool to prevent the chimpanzee, Cooper, from drowning. The chimp was immediately curious about the ropes, and within a few minutes, Cooper started diving into the six-foot-deep water to retrieve objects on the bottom of the pool. “It was very surprising behavior for an animal that is thought to be very afraid of water,” said Renato Bender. Cooper began to swim on the surface of the water some weeks later.

Suryia, the orangutan, was filmed in a private zoo in South Carolina. Suryia also possesses this rare ability to dive and swim freely up to almost 40 feet.

Cooper and Suryia both used a leg movement similar to the human breaststroke “frog kick.” They do display differences, however. Suryia moves the hind legs alternatively, while Cooper moves them synchronous. This swimming style might be due to an ancient adaptation to an arboreal life, the study suggests.

Most mammals swim using the so-called dog-paddle, which is instinctive. In contrast, humans and apes must learn to swim. With less opportunity to move on the ground, the tree-dwelling ancestors of apes developed alternative strategies to cross small rivers. They waded in an upright position or used natural bridges – losing the instinct to swim. Humans are closely related to the apes and have also lost their instinct for swimming. Unlike apes, however, humans are attracted to water and can learn to dive and swim.

“The behavior of the great apes in water has been largely neglected in anthropology. That's one of the reasons why swimming in apes was never before scientifically described, although these animals have otherwise been studied very thoroughly. We did find other well-documented cases of swimming and diving apes, but Cooper and Suryia are the only ones we were able to film. We still do not know when the ancestors of humans began to swim and dive regularly,” said Nicole Bender.

“This issue is becoming more and more the focus of research. There is still much to explore,” said Renato Bender.